SMB Server Phase 2: VFS backend build fix + integration test

- Add VfsFile: Send supertrait for Mutex compatibility
- Fix SmbServerCommand: struct → Subcommand enum with Start variant
- Fix tracing_subscriber::init() → try_init() to avoid panic when
  logger already initialized
- Fix CLI subcommand name: smb-server → smb-start (flatten naming)
- Add #[command(name = "smb-start")] for CLI disambiguation
- Fix unused variable warnings (smb_fs.rs, smb_server_backend.rs)
- Remove unused VfsFile imports (webdav.rs, scp_handler.rs)
- Integration test: Docker smbclient verified (list, upload, read)

vendor/smb2/src/msg/logoff.rs

@@ -0,0 +1,42 @@
+//! SMB2 LOGOFF request and response (spec sections 2.2.7, 2.2.8).
+//!
+//! Logoff messages request and confirm termination of a session.
+//! Both request and response contain only a StructureSize field and a
+//! reserved field, for a total of 4 bytes each.
+
+super::trivial_message! {
+    /// SMB2 LOGOFF request (spec section 2.2.7).
+    ///
+    /// Sent by the client to request termination of a particular session.
+    /// Contains only StructureSize (2 bytes) and Reserved (2 bytes).
+    pub struct LogoffRequest;
+}
+
+super::trivial_message! {
+    /// SMB2 LOGOFF response (spec section 2.2.8).
+    ///
+    /// Sent by the server to confirm that a LOGOFF request was processed.
+    /// Contains only StructureSize (2 bytes) and Reserved (2 bytes).
+    pub struct LogoffResponse;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    super::super::trivial_message_tests!(
+        LogoffRequest,
+        logoff_request_known_bytes,
+        logoff_request_roundtrip,
+        logoff_request_wrong_structure_size,
+        logoff_request_too_short
+    );
+
+    super::super::trivial_message_tests!(
+        LogoffResponse,
+        logoff_response_known_bytes,
+        logoff_response_roundtrip,
+        logoff_response_wrong_structure_size,
+        logoff_response_too_short
+    );
+}